1. Field of the Invention
This invention relates generally to a system for processing digital images and more particularly to a method for obtaining a continuous tone digital image from a stored digital halftone image.
2. Description of the Prior Art
It is a generally known procedure to reverse the digital halftone process in order to approximate an original continuous tone digital image. A traditional method of reversing the halftoning process is through the application of a low-pass filter to the binary image data. The low-pass filter method, however, cannot maintain the fidelity of the edge information contained in the original image and in fact may blur edges and introduce artifacts into the continuous tone outputimage.
U.S. Pat. No. 4,630,125 to Roetling, assigned to Xerox Corporation, discloses a method of reconstructing a continuous tone image of greyscale values that have been converted to a halftone image of black and white spots. The method of reconstruction involves isolatinf each spot of a halftone image along with a neighborhood of surrounding spots, and for each neighborhood comparing a maximum screen pattern value producing a white spot with a minimum screen value producing a black spot. If the minimum screen value giving a black spot is greater than the maximum screen value giving a white spot, then the greyscale pixel value of the isolated spot is the average of the maximum and minimum screen values just described. If the minimum screen value giving a black spot is less than the maximum screen value giving a white spot, then the process is repeated after deleting that portion of the neigborhood of surrounding spots containing the maximum or minimum screen value furthest from the isolated spot.
U.S. Pat. No. 4,841,377 to Hiratsuka et al discloses a method for estimating an original continuous tone image from a stored binary image. The method involves, inter alia, setting a plurality of scanning apertures in a binary image formed of a dither matrix, selecting one scanning aperture satisfying a predetermined condition for each picture element of a continuous image to be estimated, and estimating the continuous image on the basis of the number of white or black picture elements in the scanning aperture selected.
It should be appreciated that the halftoning process includes a loss of information, and therefore, it is impossible to exactly reverse the halftoning process and achieve a continuous tone image identical to the original. However, just as a halftone or dithered digital image gives a visual impression of a grey, it should be possible to approximate the original continuous tone digital image using reconstructive methods.
A drawback of the Roetling method of reconstructing the continuous tone image from a halftone image is that the method assumes that the halftone screen parameters are known initially. In addition, the Roetling approach uses a fixed size initial neighborhood and does not select the neighborhood adaptively, for instance, according to image content.
It is an object of the present invention, therefore, to provide a new and improved method for unscreening a halftone digital image, including the process of determining the parameters of the halftone screen or dither matrix used, and the process of reducing the appearance of coarse quantization artifacts introduced during the halftoning process. It is a further object of the present invention to produce a continuous tone output image of substantially better detail than possible using prior art methods. It is also an object of the present invention to provide a flexible unscreening algorithm capable of incorporating additional functionality so as to enable customization of the unscreening process according to the output effects desired, for example, modification of the tonal reproduction curve (TRC) used to produce the continuous tone output image.
Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.